No geological phenomenon assails our senses quite like a volcanic eruption. Stay close enough, and you can hear the explosion, see the fire fountaining, smell the gases, feel the ground tremble, taste the ash in your mouth. "I think that is why volcanoes are so cool to grade school kids," says Chris Nye, a volcanologist at the Alaska Volcano Observatory in Anchorage. "And scientifically, volcanoes are interesting because they bring you information about the interior of the planet, down to 60 miles or more, and help you study the evolution of the planets, on a human time scale. Mostly in geology you think of processes taking thousands or millions of years."

All volcanoes are born when hot magma rises to the surface, infiltrates a weak spot in the Earth's outer crust, and breaks through. Most of the 600-plus active volcanoes on Earth are associated with the boundaries of the tectonic plates, the seven great plates that carry the oceans and continents.They are especially common in subduction zones, which occur when one plate dips beneath another. As the plate dives into the mantle -- the layer of hot, flexible rock on which the plates glide -- it gradually is heated. That releases fluids which heat the overlying rock, producing blobs of molten rock that rise to the surface. The molten rock -- or magma -- collects in weak patches of crust, in structures called magma chambers. If the pressure in the magma chamber builds high enough, the magma will erupt. A volcano is born. (See Volcanic Eruption animation, 10K. You will need the free Flash plug-in to view this animation.)

Two "dome fountains" during Kilauea’s east rift eruption on June 29, 1970.

Geophysicists used to think that the movement of magma from the base of the crust, and then out of the volcano in an eruption, took centuries or more. In the last decade or so, however, researchers have found evidence that volcanism is a much more dynamic, rapid process.

"If you think of a freshman geology textbook, there is always a picture of a volcano with a magma chamber, large compared to the volcano. I think that most people walk away from that with the sense that these magma chambers exist for millennia and that -- for some reason never quite spelled out -- once in a while they burp out a bit of magma, which becomes lava when it comes to the surface," says Nye. "But in fact things are happening much more quickly than that, on time scales of days or weeks for magma to move from a magma chamber and then up to the surface to erupt."

The world's most explosive -- and devastating -- volcanic eruptions usually occur in subduction zones, because subducting oceanic plates are soaked with water, and that water helps the overlying rock melt. Ultimately, the result is a particularly gassy magma. This andesitic magma, as it is called, is very viscous -- that is, resistant to flow, like maple syrup compared to water. Such is the case in the Cascades Range of the Pacific northwest, the home of Mount St. Helens and 14 other large volcanoes.

Andesitic magma is not explosive in and of itself. But it does impede the escape of gases out of the magma chamber. Their migration inhibited, the gases form bubbles and pockets in the magma. Eventually, the pressure of the collected gases rises so high that they blow through the magma like a cork out of a champagne bottle. The result is an explosion of gas, ash, and fiery fragments of volcanic rock.

Explosive volcanoes typically have a characteristic shape -- tall, with a steep summit, created out of alternating layers of lava and volcanic rock fragments -- known as a composite cone or stratovolcano. Many of history's most famous volcanoes -- Etna, Vesuvius, St. Helens, Fujiyama -- are stratovolcanoes. (Very rapidly formed volcanoes, like Paricutin in Mexico, are often a type known as a cinder cone, built out of layers of ejected volcanic rock. These volcanoes are typically no more than 1,000 or so feet tall, whereas stratovolcanoes can become mountains.)

Most of the damage in stratovolcano eruptions comes not from lava flow but from a phenomenon known as pyroclastic flow. A pyroclastic flow is an avalanche of ground-hugging hot rock accompanied by a cloud of ash and gas that races down the slope of a volcano. The flow can reach speeds of up to 60 miles per hour, and temperatures of nearly 1,300 degrees Fahrenheit. Pyroclastic flows cause more death and destruction than any other volcanic hazard. In 1902 on the Caribbean island of Martinique, a pyroclastic flow generated by the eruption of Mt. Pelée swept into the town of St. Pierre and incinerated 29,000 people. The devastating mudflow that killed 25,000 people in Armaro, Colombia, after the 1985 eruption of Nevado del Ruiz volcano (described in the SAVAGE EARTH program "Out of the Inferno") was triggered by a pyroclastic flow.

Pyroclastic flow and lava aren't the only hazards created by volcanic eruptions. Other dangers are lahars -- mixtures of rock fragments and water that flood down volcanoes (mudflows are one type) -- landslides, gas emissions, and ash clouds. Ash clouds are a particular problem for aircraft. They can cause engine failure, damage electrical systems, scratch the outer surface of a plane, and contaminate its interior.

The effects of a volcanic eruption can also be felt over the long term. Eruptions releasing high concentrations of sulfur-rich gas -- like the eruptions of the Philippines' Mount Pinatubo, in 1991, and Mexico's El Chichón in 1982 -- can alter global climate. The sulfur mixes with water vapor in the atmosphere to form clouds of sulfuric acid. The acid droplets both absorb incoming solar radiation and bounce it back into space. The result: lower temperatures. In the year after the eruption of Pinatubo, for example, global temperatures dipped by nearly a degree.

Of course, volcanoes aren't always associated with plate boundaries. Volcano chains like the Hawaiian islands are formed by plumes of hot mantle material that rise up from the mantle and intrude on weak parts of the crust within the interior of a plate. The plumes are called "hot spots." (See The Hot Zones animation, 8K. You will need the free Flash plug-in to view this animation.)

The composition of volcanoes that form from hot spots are often much different than subduction zone volcanoes. Typically, the magma is basaltic -- it has a lower quantity of silica -- and so it flows much more easily than andesitic magma. (Some basaltic lava flows -- in particular, flows of a ropy, smooth-skinned type of lava called "pahoehoe" -- can move downhill at speeds of over six miles per hour; the motion of a viscous lava, in contrast, is often imperceptible.)

In these volcanoes, gases are released with relative ease; as they escape, they often propel incandescent blobs of lava hundreds of feet into the air, creating spectacular fountains. Hot-spot volcanoes like those in Hawaii often form a characteristic broad, flat shape, like that of a warrior's shield, and are known as shield volcanoes.

Hawaii's Mount Kilauea has essentially been continuously erupting since 1983, which has made it an ideal test site for a new system to predict volcanic eruptions. The system, first tested by researchers from Stanford University in January 1997, uses a network of receivers hooked into the satellite Global Positioning System. By looking at the position of the receivers, which can be determined to within a fraction of an inch, researchers can determine if the ground beneath the volcano is shifting or deforming, as it would if it were filling with magma. (Other tell-tale signs of impending eruption -- such as particular changes in gas emissions and the frequency of earthquakes -- are currently being studied by researchers at other volcanoes.) In the test, researchers did see signs that the ground swelled, by as much as eight inches, in the hours before an eruption on January 30. At the time, however, their system was not working in real-time, so they didn't see the signals until after the eruption. Soon, however, they hope to be able to actually predict eruptions.